1. Field of the Invention:
The present invention relates to a motor-operated power steering apparatus with a rack and pinion mechanism.
2. Description of the Relevant Art:
One motor-operated power steering apparatus wherein assistive steering power is generated by an electric motor and applied to a rack and pinion mechanism is disclosed in Japanese Laid-Open Patent Publication No. 59-50864 (corresponding U.S. Pat. No. 4,415,054). In the disclosed motor-operated power steering apparatus, rotative power produced by the motor which is disposed coaxially with a rack shaft is converted by a ball screw mechanism to a force in the axial direction of the rack shaft for assisting the steering torque imposed by a steering wheel to reduce steering efforts.
According to the earlier motor-operated power steering apparatus referred to above, as shown in FIGS. 3 and 5 of the accompanying drawings, seal members 13 are disposed respectively on the opposite ends of a gear casing 11 to seal the space in the gear casing 11 in which a gear mechanism 14 and a ball screw mechanism 15 are housed, so that the gear mechanism 14 and the ball screw mechanism 15 are resistant to dust and water. The gear mechanism 14 comprises rack teeth 14a and a pinion gear 14b in mesh therewith. The ball screw mechanism 15 comprises a helical groove 15a defined in a rack shaft 12 and a plurality of balls (not shown) rollingly riding in the helical groove 15a.
The seal members 13 on the opposite ends of the gear casing 11 are held in sliding contact with the peripheral surface of the rack shaft 12. Therefore, the rack shaft 12 must have opposite end portions each having an axial dimension L for sliding engagement with one of the seal members 13. Accordingly, the rack shaft 12 and the gear casing 11 are long and, as a result, the power steering apparatus is heavy.
More specifically, in order to prevent the rack teeth 14a and the helical groove 15a from contacting the seal members 13 or from projecting out of the gear casing 11 when the rack shaft 12 is axially moved a maximum distance L/2 from the neutral position thereof (i.e., the neutral position of the steering wheel), each end portion of the rack shaft 12 must be L/2 long on each side of the seal member 13 at the time the rack shaft 12 is in the neutral position. For this reason, the rack shaft 12 and the gear casing 11 are long and heavy. L indicates the distance which the rack shaft 12 is moved axially when the steering wheel is turned from one maximum-steering-angle position to the other maximum-steering-angle position, i.e., from lock to lock.